A 300 mm lens is not so very much yet. So that you know what to expect, here is my shot of the planet Jupiter with four of its moons. My lens is a Sigma 70-300 mm cheap lens, but for this sample it does not matter, the 300 mm focal length is in question here:
^^ Jupiter and moons cropped to show 100% pixels inline.
^^ Here the full resolution photo. My camera has a 16 MP APS-C sensor.
Click here for full size photo (4912x3264).
To get closer you'll need a teleconverter in between the lens and your camera.